Multiple myeloma (MM) is an incurable neoplasm characterized by devastating and progressive bone destruction. Despite recent advances in the first-line treatment of MM, almost all patients eventually relapse, become chemoresistant, and die of the disease. Clinical studies with Bortezomib have validated the proteasome as a therapeutic target for the treatment of MM and non-Hodgkin's lymphoma. However, significant toxicities have restricted the intensity of Bortezomib dosing and its clinical efficacy has been hampered by the emergence of resistance. There is great need for methods to target the delivery of novel, effective cytotoxic agents specifically to bone, where myeloma cells reside. The long-term goal of this proposal is to develop novel and innovative strategies aimed at selectively delivering therapeutic agents to the bone microenvironment for effective management of MM. Our recent studies of the bisphosphonate-proteasome inhibitor (BP-PI) conjugates indicate that the stability of the amide linkage does not permit efficient release of the drug and that the presence of the highly charged bisphosphonate moiety prevents cellular uptake of the conjugates, resulting in diminished cytotoxicity. In this application, our goal is to synthesize and evaluate the efficacy of a new series of tumor-activated bone-targeted proteasome inhibitors using a dual targeting strategy that targets matrix metalloproteinases (MMPs) produced by myeloma cells in the bone microenvironment to cleave a unique bisphosphonate-heptapeptide linker. It will enable us to investigate the potential of exploiting MMP-9 to activate prodrugs rationally designed to target MM cells as a new tumor-specific therapy. Our hypothesis is that by linking bisphosphonates to proteasome inhibitors through an MMP-cleavable heptapeptide, 1) we can target these compounds to deposits of myeloma cells in the bone microenvironment, 2) MMPs produced by the tumor cells will effectively cleave the bisphosphonate to release the warhead and achieve more effective concentrations of proteasome inhibitors locally, and 3) the tumor-activated prodrugs will selectively kill tumor cells and prevent or reduce dose-limiting systemic toxicity of pharmaceuticals. The specific aims of the proposal are i) To design and synthesize novel MMP-9 cleavable BP-heptapeptide-PI conjugates as a means of selective delivery of PS-341 to MM cell in bone microenvironment, and ii) To study the proposed compounds in vitro and in vivo. Together, these screens will provide validation for bone-targeted tumor-activated prodrugs as an effective treatment modality for treatment of MM and other bone metastatic cancers.